Wiring systems based on optical cables are becoming of ever greater interest for backbone wiring systems and in conjunction with Fast Ethernet and Gigabit Ethernet. Signals are transmitted in optical fibers, which are also referred to as optical waveguides, uni-directionally by means of light pulses, that is to say in only one direction, for which reason at least two optical fibers are typically used per cable. The light pulses are generally injected into the fiber by means of a laser diode or a light-emitting diode.
The advantages of optical data transmission are the high achievable transmission rate and the long range, as well as the insensitivity to electromagnetic radiation, security against eavesdropping and resistance to heat and weather influences. Because the inner conductors are thin, optical cables can also be laid flexibly.
In addition to pure glass fibers, optical fibers composed of polymers have also been widely used, because their costs are lower. Optical fibers composed of polymers are referred to as POFs (Plastic Optical Fibers), and are pure plastic fibers which are composed of a transparent core and sheath, with the sheath having a lower refractive index than the core material. Polymers such as polymethylmethacrylate or polycarbonate are used as the core material. In addition to the purely polymer fibers, polymer optical fibers also include hybrid fibers which have a combination of glass fiber and plastic sheathing, for example HCS (Hard Polymer Cladded Silica) fibers.
The distance which can be bridged directly by optical waveguides is limited by various dispersion and scatter effects. The distance is a function of the bandwidth to be transmitted and is thus typically quoted as the bandwidth/length product. In comparison to glass fibers, polymer fibers such as POF or HCS have considerably higher attenuation levels. POF and HCS fiber systems are already reaching the limits of their technical capabilities when being operated over path lengths of 50 or 100 m and at data rates of 100 Mbit/s or 125 MBd. In order to ensure reliable data transmission, diagnosis of the optical fibers of the transmission path is now a conventional method.
One method for diagnosis of optical waveguide paths, in particular for optical diagnosis in the case of Interbus, is known for example from DE 42 17 899 C2. The method described there is used for system optimization of optical waveguide transmission paths during commissioning and provides for the optical transmission power level of a transceiver to be varied until the optical signal received at the opposite end corresponds to the system requirements.
EP 1 227 604 A2 discloses a method in which the instantaneous level margin with respect to the sensitivity limit, that is to say between the instantaneous transmission power of the transmitter and the instantaneous sensitivity limit of the receiver, is determined for an optical transmission path.
The aim of optical diagnosis according to the prior art is to diagnose attenuation of optical connecting paths in order to deduce whether transmission is reliable. This is actually adequate for low-bit-rate systems. However, at data rates above 100 Mbit/s, in the case of polymer (POF) and HCS fibers, the bandwidth of the cable becomes the limiting factor for error-free transmission, rather than the cable attenuation. However, this cannot be tested automatically by means of the methods that are now known. A further disadvantage of known systems is that the fiber type that is used, such as POF or HCS which can be operated on the same interface, cannot be determined automatically. This also has a disadvantageous effect on the attenuation diagnosis since different fiber types also have a different attenuation response, and the diagnosis data must therefore be assessed differently. Nowadays, the fiber type must typically be stated manually in the diagnosis software.